Despite　the　fact　that　concrete　pore　pressures　play　an　important　role　in　structural　integrity　in　realistic　fire　scenarios,　there　are　still　few　reports　on　understanding　the　evolution　of　pore　pressures　during　all　fire　phases,　especially　during　constant　temperature　and　natural　cooling.　The　current　gap　in　understanding　still　exists　due　to　the　past　focus　on　the　heating　phase.　This　study　investigated　pore　pressure　variations　in　natural　fiber-reinforced　self-compacting　concrete　with　varying　dosages　under　high-temperature　conditions,　expanding　upon　previous　research　that　focused　solely　on　pore　pressure　measurements　during　the　heating　phase.　Novel　observations　were　incorporated　for　both　constant-temperature　and　natural　cooling　stages,　with　particular　emphasis　on　the　emergence　and　dissipation　mechanisms　of　pore　pressure　arising　from　thermal　expansion　mismatch　between　the　matrix　and　vapor.　During　the　heating　phase,　pore　pressure　was　primarily　attributed　to　the　vapor　pressure　generated　by　the　evaporation　and　expansion　of　free　water　in　cementitious　materials　and　decomposition　water　from　hydration　products,　accompanied　by　in-depth　analysis　of　vapor　source　identification　and　pore　formation　dynamics.　Notably,　a　phenomenon　contradicting　conventional　expectations　emerged　during　the　constant-temperature　phase:　The　persistent　thermal　expansion　discrepancy　between　matrix　pores　and　vapor　resulted　in　incomplete　vapor　release,　thereby　inducing　secondary　pore　pressure　development.　Meanwhile,　the　maximum　pore　pressure　decreased　from　1.91　MPa　to　1　MPa　as　the　fibre　volume　doping　increased　from　0　to　0.3　%.　To　elucidate　these　mechanisms,　systematic　validation　was　conducted　through　thermal　expansion　testing,　mass　variation,　FTIR,　and　water　vapor　adsorption　experiments.　During　the　cooling　phase,　synchronized　temperature　reduction　of　internal　moisture　decreased　pore　pressure,　while　vapor　adsorption　from　external　environment　by　the　matrix　led　to　further　mass　increase.　©　2025　Elsevier　Ltd